Note that Airplay is very sensitive to the throughput and network queueing. Once a connection is established with an endpoint (in my case AppleTV) the data transfer is via UDP. I find that from my MacBook Pro through a very robust home network (wireless N 450bps verified, and gigabit wired to AppleTV switch) I can easily cause Airplay dropouts with the slightest browsing (or Time Machine, etc. causing any other material amounts of network traffic) on my MBP while it is playing music, regardless of whether I'm using iTunes, MC18 or any other music software. FWIW, all my music is on a Synology NAS, so everything must be picked up off the network before being sent back out via Airplay.
That said, I love the convenience of Airplay. But for quality I plug in the optical to the MBP.